induced pluripotent stem cells in the study of neurological diseases

Introduction

Neurological disorders account for 6.3% of the global

burden of disease [1,2] and are expected to rise in

incidence as the world population ages. Nevertheless,

there are few eff ective drug treatments, probably due to a

lack of human disease models and poor understanding of

fundamental disease mechanisms.

Most neurological disorders are caused by dysfunction

and eventual loss of specifi c, highly specialized sub popu-

lations of neuronal and/or glial cells. As human neurons

and glia are not readily available, pathophysiological

studies have been traditionally limited to genetically

engineered animal models or cell lines less relevant to

disease pathophysiology, such as skin fi broblasts or

immortalized cell lines. While these surrogate models

provide some insight into disease mechanisms, their

genotype and phenotype diff er considerably from those

of disease-aff ected cells in vivo. Th is is particularly true

for diseases where gene dosage seems to play an impor-

tant role, such as in superoxide dismutase 1 (SOD1)-

associated familial amyotrophic lateral sclerosis (ALS) [3]

and Charcot-Marie-Tooth disease type 1A, caused by

PMP22 duplication [4,5]. To study these conditions,

multiple copies of the mutant gene are inserted into

animal models, artifi cially creating a phenotype that

resembles the human disease but not necessarily

recapitulating the biological mechanisms behind it.

Another example of a human disease that does not

readily translate into animal models or traditionally used

cell lines is spinal muscular atrophy (SMA), caused by

deletions of the SMN1 gene [6]. In humans, the disease

phenotype is modulated by the expression levels of

SMN2, which is absent in mice and other species

commonly used as disease models [7,8]. In addition, even

though SMN1 is ubiquitously expressed in all cells, motor

neurons are primarily aff ected in SMA patients. Disease

models should therefore refl ect a specifi c pathophysio-

logical context and cellular networks that exist in the

disease-relevant cells.

Th e recent development of induced pluripotent stem

(iPS) cell technology has provided a new paradigm for the

generation and study of human disease-specifi c neuronal

and glial cells relevant for investigating neuro logical dis-

orders (Figure 1). Because this technology makes physio-

logically relevant, pathological cells avail able in limitless

amounts, it will probably prove to be a more translational

approach to study nervous system function and disease

and to screen potentially therapeutic compounds more

reliably. Here, we review the recent developments in the

use of iPS cells to model neurological diseases and discuss

the major challenges in moving the fi eld forward.

Induced pluripotent stem cells: generation and

diff erentiation to neurologic disease-relevant cell

lineages

Embryonic-like iPS cells capable of diff erentiating into a

variety of cells in the body can be derived from somatic

Abstract

Five years after their initial derivation from mouse

somatic cells, induced pluripotent stem (iPS) cells

are an important tool for the study of neurological

diseases. By off ering an unlimited source of patient-

specifi c disease-relevant neuronal and glial cells, iPS

cell-based disease models hold enormous promise

for identifi cation of disease mechanisms, discovery of

molecular targets and development of phenotypic

screens for drug discovery. The present review focuses

on the recent advancements in modeling neurological

disorders, including the demonstration of disease-

specifi c phenotypes in iPS cell-derived neurons

generated from patients with spinal muscular atrophy,

familial dysautonomia, Rett syndrome, schizophrenia

and Parkinson disease. The ability of this approach

to detect treatment eff ects from known therapeutic

compounds has also been demonstrated, providing

proof of principle for the use of iPS cell-derived cells in

drug discovery.

© 2010 BioMed Central Ltd

Induced pluripotent stem cells in the study of neurological diseasesMario A Saporta1,2*, Marica Grskovic1 and John T Dimos1*

R E V I E W

*Correspondence: [email protected]; [email protected], Inc., 951 Gateway Blvd, South San Francisco, CA 94080, USA

Full list of author information is available at the end of the article

Saporta et al. Stem Cell Research & Therapy 2011, 2:37 http://stemcellres.com/content/2/5/37

© 2011 BioMed Central Ltd

cells by the forced expression of defi ned factors [9-11].

Distinct factors, and strategies to induce their expression,

have been employed for the generation of iPS cells from a

number of human tissues using an array of approaches

with varying degrees of effi ciency [12]. To date, however,

most patient iPS cell lines have been derived by retroviral

transduction of dermal fi broblasts due to their accessi-

bility and relatively high effi ciency of reprogramming.

iPS cells can be coaxed into specifi c cell types by mani-

pu lation of the culture environment. Growth factors,

small molecules and extracellular matrix proteins can be

applied in a sequential manner to emulate the normal

development of the cell lineage of interest. Using this

approach, investigators have been able to diff erentiate

human pluripotent cells into lineages necessary for

model ing neurological diseases, including cholinergic

[13,14], glutamatergic [15] and dopaminer gic neurons

[16,17], astrocytes [13], oligodendrocytes [18] and Schwann

cells [19,20].

Spinal cord cholinergic motor neuron diff erentiation is

one of the better studied among the aforementioned cell

types and follows the same steps described during

normal embryonic development [21]. Th e fi rst step in

diff erentiating iPS cells into neurons is inhibition of

pathways such as those of transforming growth factor

beta and bone morphogenetic protein [22]. iPS cells

diff erentiate to neuroepithelia usually within a few days

of compound treatment and assume a neural tube-like

rosette morpho logy. Th is primitive neuroepithelium can

be patterned to ventral spinal progenitors by treatment

with retinoic acid and sonic hedgehog or one of its

signaling agonists. Retinoic acid is the main signal for

Figure 1. Human induced pluripotent stem cells can be diff erentiated into cell types to study neurological disorders. Human induced

pluripotent (iPS) stem cells can be diff erentiated into cell types relevant for the study of neurological disorders. Somatic cells from patients with

neurological disorders can be reprogrammed into pluripotent stem cells, which in turn can be diff erentiated into distinct neuronal and glial

cell types, thus off ering a human cell platform for mechanistic studies and high-throughput screening for diseases of the central and peripheral

nervous system.

Somatic cells

Reprogramming

iPS cells

Differentiation

NeuronsGlia

Phenotype characterization• ell mor olo r anelle tra ic in

• lectro siolo Gene e ression

• onnecti it Neuronal-glia interactions

Mechanistic studies• arget i enti ication at a s molecules)

Drug discoveryHigh throughput screening

Cell replacement

therapy


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neurons to assume a caudal (spinal cord) profi le, while

sonic hedgehog deter mines a ventral (motor) identity.

Further diff erentiation to mature spinal motor neurons

can then be accom plished by addition of specifi c factors,

such as brain-derived neurotrophic factor and glial cell-

derived neurotrophic factor, both of which promote

axonal elongation [23]. Th is process usually takes around

3 to 6 weeks depending on the specifi c protocol, and can

be monitored using a set of markers including PAX6

(neuro epithelia), OLIG2 (motor neuron progenitors),

ISLET1/2 and HB9 (motor neurons), and acetylcholine

transferase and synapsin (mature motor neurons), among

others. Alternative approaches including the generation of

embryoid bodies as an intermediate step have also been

described [24]. Consistent with what is seen in normal

development, glia cell diff erentiation only occurs after a

prolonged time in culture, usually between 6 and 8 weeks.

Modeling neurological diseases using iPS cells

Identifi cation of a disease-relevant phenotypic diff erence

between cells derived from patients and from healthy

individuals is one of the most challenging aspects of

using iPS cells for disease modeling. Th is is particularly

relevant for diseases where causative cellular patho-

physiology is not clear, such as familial ALS or Alzheimer

disease. Even though iPS cells have been derived from

patients with a number of neurological diseases

(summarized in the next sections and in Table 1), initial

work has focused on modeling neurodevelopmental dis-

orders – in particular, those with known genetic causes.

Modeling genetically complex, late-onset diseases is

probably more challenging, and may require exposing the

cells to biological, chemical or environmental stressors to

reveal pathological phenotypes. Th e examples discussed

below demonstrate the value of iPS cell-based models for

identifi cation of disease mechanisms, discovery of mole-

cu lar targets and development of phenotypic screens for

drug discovery.

Monogenic early-onset disorders

Spinal muscular atrophy

SMA (OMIM: 253300) is an autosomal recessive disease

that aff ects one in every 6,000 to 10,000 live births,

making it the most common neurogenetic disorder of

infancy. SMA is caused by a decrease in levels of survival

of motor neuron (SMN) protein due to deletions of the

SMN1 gene. Even though SMN protein is ubiquitously

expressed, its defi ciency leads to a loss of motor neurons

of the spinal cord ventral horns and consequent dener-

vation of axial and limb muscles, represented clinically by

muscle atrophy and weakness, dysphagia and respiratory

failure in severe cases [25]. Th e clinical phenotype of

SMA is modulated by the expression level of SMN2, a

paralog almost identical to SMN1. SMN2 generates low

levels of the SMN protein that are not suffi cient to

prevent loss of motor neurons.

Past studies have largely relied on animal models or

unaff ected cell types such as patients’ fi broblasts,

providing limited insight into the disease mechanism and

yielding ineff ective drug treatments. In the fi rst proof-of-

principle study using iPS cells to model a disease, Ebert

and colleagues generated iPS cells from a SMA patient

and used them to derive motor neurons [14]. Interest-

ingly, the authors found comparable size and number of

motor neurons at 4 weeks of diff erentiation between the

SMA and control cultures. By week 6, however, the SMA

motor neurons were selectively reduced in number and

size when compared with the control cells – suggesting

that SMA motor neurons developed normally, but were

more susceptible to degeneration. Th e authors identifi ed

a reduction in SMN aggregates (also termed gems) in

SMA motor neurons, consistent with the reduced levels

of SMN in these cells. Th e administration of valproic acid

and tobramycin led to the increase of gems in SMA iPS

cells. While this study did not show whether these

compounds can elevate SMN levels or rescue the loss of

patient-derived motor neurons, it provided an important

validation for the utility of iPS-derived patient cells to

model disease.

Familial dysautonomia

Familial dysautonomia (FD) is one of the hereditary

sensory and autonomic neuropathies (type III, or Riley–

Day syndrome; OMIM: 223900). FD is an autosomal

recessive disorder almost exclusive to individuals of

Eastern European Jewish origin, aff ecting one in every

3,600 live births in this population. Clinically, it is

characterized by feeding diffi culty, alacrimia, orthostatic

hypotension without compensatory tachycardia, and

decreased pain and temperature perception. FD is usually

fatal, with only one-half of the patients reaching adult-

hood, even with the best standard of care [26].

FD is caused by mutations in the IKBKAP gene [27]

that lead to reduced transcriptional elongation of several

target genes, some of which are required for cell motility

[28]. In a recent study, Lee and colleagues generated iPS

cell lines from three patients with FD and demonstrated

several disease-relevant features specifi c to the patients’

cell lines, including misregulated inhibitor of kappa light

polypeptide gene enhancer in B cells, kinase complex-

associated protein (IKBKAP) expression, defective

neuronal diff erentiation and a decrease in FD neural crest

precursor migration [29]. By comparing gene expression

profi les of healthy and patient-derived neural crest pre-

cursors, genes involved in peripheral neurogenesis and

neuronal diff erentiation were found to be diff erentially

expressed in FD cells, providing insight into the mole-

cular mechanism(s) of the disease.


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Table 1. Neurological and psychiatric diseases where iPS cells have been derived from aff ected individuals

Phenotype of iPS-derived Therapeutic responseDisease Reference Molecular defect cells (compound)

Down syndrome Park and colleagues [48] Trisomy 21 ND ND

Lesch–Nyhan syndrome

(carrier state)

Park and colleagues [48] Heterozygous point mutations

in HPRT1 gene

ND ND

Khan and colleagues [55]

Huntington disease Park and colleagues [48] Trinucleotide expansion in

HUNTINGTIN

ND ND

Zhang and colleagues [50] Enhanced caspase 3/7 activity

after growth factor withdrawal

ND

Duchenne and Becker

muscular dystrophy

Park and colleagues [48] Mutations in DYSTROPHIN ND ND

Kazuki and colleagues [49]

Familial amyotrophic

lateral sclerosis

Dimos and colleagues [13] Mutations in SOD1 gene ND ND

Boulting and colleagues [34]

Spinal muscular atrophy Ebert and colleagues [14] Mutations in SMN1 gene Reduced size and number of

motor neurons, reduced SMN

protein in iPS cells

Increased SMN gem number

in SMA iPS cells (valproic acid,

tobramycin)

Familial dysautonomia Lee and colleagues [29] Partial skipping of exon 20 of

IKBKAP, reduced IKAP protein

Decreased expression of genes

involved in neurogenesis and

neuronal diff erentiation; defect in

neural crest migration

Increase in the percentage of

diff erentiating neurons and in

the expression of peripheral

neuron markers (kinetin)

Fragile X syndrome Urbach and colleagues [56] Trinucleotide (CGG)

expansion, silencing of FMR1

ND ND

Angelman syndrome and

Prader–Willi syndrome

Chamberlain and colleagues

[51]

Chromosome 15q deletion

(imprinting disorders)

ND ND

Yang and colleagues [52]

Parkinson disease Park and colleagues [48] Unknown (sporadic) ND ND

Soldner and colleagues [16]

Swistowski and colleagues

[17]

Hargus and colleagues [37]

Nguyen and colleagues [38] Mutations in LRRK2 Increased expression of stress-

response genes, increased

α-SYNUCLEIN levels and

oversensitivity to stress agents by

dopaminergic neurons

ND

Seibler and colleagues [42] Mutations in PINK1 Impaired recruitment of Parkin

to mitochondria, increased

mitochondrial copy number,

upregulation of PGC-1α in

dopaminergic neurons

Phenotype corrected by

expression of wildtype PINK1

Rett syndrome Hotta and colleagues [57] Mutation in MECP2 Decreased synapse number,

reduced spines, increased LINE1

retrotransposon mobility

Increase in glutamatergic

synapse number (IGF1);

increase in MeCP2 protein

levels and glutamatergic

synapse numbers (gentamicin)

Marchetto and colleagues

[15]

Muotri and colleagues [33]

Schizophrenia Brennand and colleagues [47] Unknown Reduced neurite density,

neuronal connectivity and

glutamate receptor expression;

altered gene expression of

components of the cyclic AMP

and WNT signaling pathways

Increase in neuronal

connectivity and glutamate

receptor expression (loxapine)

ND, not demonstrated; FMR1, fragile X mental retardation 1; HPRT1, hypoxanthine phosphoribosyltransferase 1; IGF1, insulin-like growth factor 1; IKBKAP, inhibitor of kappa light polypeptide gene enhancer in B cells, kinase complex-associated protein; iPS, induced pluripotent stem; LRRK2, leucine-rich repeat kinase 2; MECP2, methyl CpG binding protein 2; PINK1, PTEN-induced putative kinase 1; SMA, spinal muscular atrophy; SMN, survival of motor neuron; SOD1, superoxide dismutase 1.


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Using iPS cell-derived neural crest cells as a drug

screening platform, a partial rescue of the disease pheno-

type was achieved after administration of kinetin, a plant

hormone previously shown to reduce levels of the mutant

IKBKAP splice form in FD-derived lymphoblast cell lines.

Kinetin treatment of patient’s cells signifi cantly reduced

the mutant IKBKAP splice form and increased the

number of diff erentiating neurons; however, the level of

increased IKBKAP did not lead to rescue of cell motility.

Even though the identifi ed compound only partially

rescued the disease phenotype in this cellular model of

FD, this study demonstrates the value of patient cell-

based disease models for drug discovery using pheno-

typic screens, as well as for identifying novel molecular

targets and disease mechanisms.

Rett syndrome

Rett syndrome (OMIM: 312750) is an X-linked autism

spectrum disorder characterized by stagnation of develop-

mental skills starting between 6 and 18 months of age,

followed by developmental regression, hypotonia, seizures

and autistic behavior. Aff ecting one in 10,000 to 20,000

females [30], it is caused by mutations in methyl CpG

binding protein 2 (MeCP2), a protein involved in epi-

genetic and transcriptional regulation of a number of

genes [31,32].

In a recent study, Marchetto and colleagues developed

iPS cell lines from four female Rett patients, diff erentiated

them into neurons and compared them with neurons

derived from healthy individuals [15]. While no

diff erences were observed in neurogenesis, mature Rett

neurons were smaller with fewer dendritic spines and less

glutaminergic excitatory synapses. Of note, this pheno-

type could be modulated by overexpression or knock-

down of MeCP2 in neurons derived from control iPS

cells, suggesting that MeCP2 is a rate-limiting factor in

determining the glutaminergic synapse number in

human neurons. Neurons derived from Rett iPS cells also

demonstrated reduced frequency of calcium oscillations

and spontaneous postsynaptic currents, suggesting a

defi ciency in neuronal network connectivity. Similarly to

the FD study, the authors identifi ed compounds that

partially rescued the disease phenotype in patient-

derived cells. Th e same group has recently used iPS cells

from Rett patients to investigate the role of MeCP2 in

modulating long interspersed nuclear elements in

neurons, providing yet another example of examining

disease mechanisms in patient iPS cell-based models

[33].

Late-onset disorders

Amyotrophic lateral sclerosis

ALS (or Lou-Gehrig’s disease) is the most common

motor neuron disease, with a prevalence of one to two

per 100,000 worldwide. ALS is characterized by progres-

sive loss of upper (cortical) and lower (spinal cord) motor

neurons, with consequent spasticity, hyperrefl exia and

progressive weakness and muscle atrophy. It is a fatal

disease with a mean overall survival between 3 and

4 years after presentation. Around 10% of cases have a

genetic etiology, and animal models have been created

based on genes identifi ed in families with ALS. Unfor tu-

nately, no signifi cant drug development has successfully

translated from these studies into clinical practice.

In the fi rst work to demonstrate that patient-specifi c

iPS cells could be diff erentiated into motor neurons,

Dimos and colleagues developed iPS cell lines from two

patients with familial ALS caused by a SOD1 point

mutation [13]. Of note, the patients were over 80 years

old at the time of the study, demonstrating that iPS cells

could be successfully generated even from mature skin

fi broblasts of the elderly and diff erentiated into spinal

motor neurons. Recently, Boulting and colleagues estab-

lished a test set of 16 iPS cell lines from fi ve healthy

controls and two patients with SOD1 familial ALS, and

demonstrated that all lines showed comparable effi ciency

in the generation of electrically active motor neurons

[34]. Th e study found line-to-line phenotypic diff erences

between distinct iPS cell lines; however, pair-wise

comparisons did not reach statistical signifi cance and

con cor dance between lines diff erentiated in two indepen-

dent laboratories was high, suggesting that the iPS cell

platform is reproducible enough to allow for detection of

consistent disease-specifi c phenotypes. Although an ALS

disease phenotype in patient-derived iPS cells has yet to

be demonstrated, iPS cell methodology enables us to

create motor neurons from familial and sporadic ALS

patients, and to identify common and diverse cellular

disease phenotypes in diff erent patients.

Parkinson disease

Parkinson disease (PD) is the second most common

neurodegenerative disorder, aff ecting more than 6 million

people worldwide [35]. It is characterized by selective

loss of dopaminergic neurons in the substancia nigra pars

compacta of the midbrain. PD is clinically defi ned by

resting tremor, reduced spontaneous movements (bradi-

ky nesia), rigidity and postural instability. A group of non-

motor PD-related symptoms has been increasingly

recognized [36], suggesting that other neuronal cell types

may also be aff ected. Although PD is a treatable condi-

tion, neurodegeneration progresses despite sympto matic

control, worsening symptoms and eventually reduc ing

therapeutic effi cacy. Dopaminergic neurons, the main

cell population aff ected by PD, have been diff eren tiated

from patient-derived iPS cells [16,17,37,38]. Th ese

neurons were successfully transplanted into rat brains,

integrated to the neuronal circuitry, survived in


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signifi cant numbers 12 weeks after transplantation and

improved clinical phenotype as defi ned by a reduction of

ampheta mine rotational asymmetry [17,37], closely

replicating previous experiments using human

embryonic stem cell-derived dopaminergic neurons

[39-41].

In a recent study, iPS cells were generated from a

patient with a homozygous point mutation in the leucine-

rich repeat kinase-2 (LRRK2) gene, the most common

cause of familial PD [38]. Dopaminergic neurons derived

from these iPS cells demonstrated increased expression

of stress-response genes, including HSPB1, NOX1 and

MAOB, increased α-SYNUCLEIN levels and oversensi-

tivity to stress agents, such as peroxide and 6-hydroxy-

dopamine. Seibler and colleagues recently derived iPS

cells from patients with familial PD caused by mutations

in the PTEN-induced putative kinase (PINK1) gene [42].

PINK1 is an outer mitochondrial membrane protein

believed to regulate the translocation of PARKIN, another

protein associated with familial PD, into damaged

mitochondria. Patient iPS cell-derived dopaminergic

neurons exhibited mitochondrial dysfunction that was

alleviated by introduction of wildtype PINK1. Taken

together, these data suggest that key features of PD

pathophysiology could be recapitulated using the iPS cell

approach. Potential disease mechanisms identifi ed in

dopaminergic neurons derived from iPS cells of familial

PD patients could be further studied in cells derived from

patients with sporadic forms of PD to help establish

common downstream pathways amenable to therapeutic

intervention.

Neuropsychiatric disorders

Schizophrenia

Schizophrenia is a devastating neuropsychiatric disease

characterized by long duration of symptoms, delusions,

lack of motivation, reduction in spontaneous speech and

social withdrawal, and few aff ective symptoms [43].

Disease onset is usually in adolescence and early adult-

hood, which causes signifi cant human and fi nancial

burden to patients, family and society as a whole [44].

Th e pathophysiology of schizophrenia is complex, includ-

ing environmental as well as strong genetic components

[45]. As with other neuropsychiatric conditions, genera-

tion of reliable animal models is limited and problematic

[46]. A recent study demonstrated disease-specifi c pheno-

types in iPS cell-derived neurons from four patients with

schizophrenia, including reduced neurite density, neuronal

connectivity and glutamate receptor expression, and

altered gene expression of components of the cyclic AMP

and WNT signaling pathways [47]. Of note, both

neuronal connectivity and gene expression abnormalities

were improved after a 3-week treatment with the anti-

psychotic loxapine.

Other neurological diseases

iPS cells have also been generated from patients with

Duchenne and Becker muscular dystrophy [48,49],

Huntington disease [48,50], and the genomic imprinting

disorders Angelman syndrome and Prader–Willi syn-

drome [51,52]. Although the resultant iPS cell lines

carried the basic genetic abnormality for each disorder,

no specifi c phenotype was described under standard

culture conditions. However, several fi ndings from these

studies are noteworthy. Striatal neurons derived from

Huntington disease iPS cells demonstrated enhanced

caspase 3/7 activity after growth factor withdrawal [50].

iPS cells derived from patients with Angelman syndrome

and Prader–Willi syndrome – neurodevelopmental dis-

orders caused by lack of expression of genes contained in

a specifi c region of chromosome 15, and defi ned by the

parental origin of the aff ected genetic material (imprint-

ing) – maintained the appropriate DNA methylation

imprint following reprogramming [51,52], validating the

use of the iPS cell model in the investigation of imprinting

diseases.

Recently, Kazuki and colleagues corrected the genetic

abnormality in fi broblasts from a patient with Duchenne

muscular dystrophy, due to a deletion of exons 4 to 43 of

the human dystrophin gene, using a human artifi cial

chromosome with a complete genomic dystrophin

sequence [49]. At 2.4 megabases, DYSTROPHIN is the

longest known gene, making gene replacement therapy

particularly challenging, especially for patients with long

deletions. Th e authors successfully derived iPS cells from

the corrected fi broblasts, demonstrating the potential for

combining gene therapy and iPS cell technology to

generate patient-specifi c rescued cell lines for eventual

use in cell replacement therapy.

Challenges and limitations

Despite the rapid progress in applying iPS cell technology

to disease modeling, this promising platform is still in its

infancy. Several issues remain to be tackled before iPS

cells can be used as reliable models of acquired, multi-

factorial disorders and, eventually, as treatment strategies

in regenerative medicine.

One immediate challenge is in using iPS cells to

produce relevant diff erentiated and functional cell

types. Current diff erentiation protocols attempt to

mimic embry onic specifi cation and patterning; for

example, using signaling molecules to dial in the desired

rostral/caudal and dorsal/ventral location. Th is

approach, how ever, generally results in a heterogeneous

cell population. While these mixed populations could

be considered co-cultures in which, particularly,

neurons are more amenable to long-term maturation

and survival, they also present a possible challenge to

phenotype identifi cation.


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Simple biochemical and gene expression analyses

cannot be performed across cultures without careful

normalization for cell types and their proportions

present, which may limit the study of conditions exclu-

sively or preferentially aff ecting one cell type. However,

approaching the diff erentiated culture similarly to a

primary explant culture, such as dorsal root ganglia

cultures where multiple cell types coexist, may be a useful

strategy. In this approach, the heterogeneity of diff eren-

tiated cultures is turned into an advantage where the cell

type of interest can be studied within a broader milieu;

for example, motor neurons with spinal cord inter-

neurons and glial cells.

Th e use of cell type-specifi c reporter genes allows for

identifi cation and characterization of the target cell while

preserving functionally meaningful interactions between

neuronal and non-neuronal cells. Recently, new tech-

niques to introduce reporter genes into cells have become

available, including bacterial artifi cial chromosomes with

fl uorescent reporters [53] and zinc fi nger nucleases [54].

Zinc fi nger nuclease technology allows for the effi cient

and rapid production of knockin reporter cell lines,

wherein sequences encoding fl uorescent reporter proteins

can be put under the control of any endogenous regu-

latory region. Such a labeling approach can in principle

allow for any cell type to be identifi ed or isolated, and the

insertion of multiple fl uorescent reporters in the same

line would potentially allow for cell diff erentiation,

maturation and function to be monitored in real time.

Another approach to study the cell type of interest in a

complex culture would be to isolate the desired cell type at

the end of diff erentiation using techniques such as fl uores-

cence-activated cell sorting or magnetic bead separation.

While combinatorial cell surface markers are well validated

for the hematopoietic system, however, identifying surface

markers specifi c for the target cell can be challenging, as is

the case for spinal cord motor neurons. Which of the

aforementioned strategies for analyz ing heterogeneous

cultures diff erentiated from iPS cells will prove to be the

more adequate to characterize particular disease-relevant

phenotypes is a matter for further study.

It remains unclear whether the iPS cell platform will be

able to replicate the more complex, multifactorial patho-

physiology of late-onset neurodegenerative disorders. It

is possible that in these conditions a disease-relevant

pheno type would only appear after a long quiescent

period, hindering the use of iPS cells in the study of late-

onset diseases. Diverse chemical, genetic or environ-

mental stressors could be applied in such instances,

however, in order to mature or age cells if necessary to

reveal a phenotype. Additionally, some pathophysiology

may require at least a partial recapitulation of central

nervous system architecture. For example, possible

defects in axonal transport in projection neurons might

only be recapitu lated in vitro when neurons are allowed

to extend axons of signifi cant length and complexity.

Another related issue, inherent to cell culture plat-

forms, is the inability of the iPS cell model to replicate

disease mechanism at the tissue or system levels –

including, for example, protein deposition or infl am ma-

tion. On the other hand, the possibility to study a more

isolated system may allow investigators to detect the

initial steps of a disease process, otherwise superimposed

to other subsequent responses. For example, while the

iPS platform will probably not be able to replicate the

complex anatomical and functional interactions between

the distinct cell types aff ected by PD, the recent report of

mitochondrial dysfunction in iPS cell-derived dopamin-

ergic neurons from a specifi c familial form of PD demon-

strates how this system can detect discrete cellular

dysfunc tion that could otherwise be masked by end-stage

changes in pathological specimens [42].

In spite of the challenges for harnessing its true

potential, iPS cell technology is likely to prove advan ta-

geous for building novel human disease models. Diff eren-

tiation protocols must be further improved while novel

culture conditions needed to support iPS cell-derived

cells and investigate their phenotypes are developed.

Conclusions

Th e development of iPS cell technology is opening a new

avenue for the study of human, disease-specifi c, neuronal

and glial cells that promises to revolutionize the neuro-

science fi eld. Since the publication of Takahashi and

Yamanaka’s seminal paper 5 years ago [9], iPS cell lines

from more than a dozen distinct neurodevelopmental

and neurodegenerative diseases have been established

and specifi c disease phenotypes are starting to emerge.

Future studies will probably focus on validating these

disease phenotypes in platforms that will allow for the

screening of therapeutic compounds and the discovery of

biologic mechanisms underlying neurological diseases.

Th e widespread availability of human disease-specifi c

cells will allow investigators the unprecedented oppor-

tunity to conduct mechanistic studies and determine

causation in a human model system, rather than just

correlation. Th is will allow in vitro phenotypes to be

linked to disease pathology, enabling a better under-

standing of therapeutic manipulations that might lead to

a disease-modifying eff ect.

Developing and validating new techniques to re-

program somatic cells into iPS cells without viral inte-

gration and to correct genetic abnormalities ex vivo are

the next step in the eff ort to apply iPS cell technology in

regenerative medicine, and are currently an active area of

research. One can envision a near future where iPS cells

will be used as a screening tool for personalized medicine

and as a reservoir for cell replacement therapy.


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Abbreviations

ALS, amyotrophic lateral sclerosis; FD, familial dysautonomia; IKBKAP, inhibitor

of kappa light polypeptide gene enhancer in B cells, kinase complex-

associated protein; iPS, induced pluripotent stem; LRRK2, leucine-rich repeat

kinase 2; MECP2, methyl CpG binding protein 2; PD, Parkinson disease; PINK1,

PTEN-induced putative kinase 1; SMA, spinal muscular atrophy; SMN, survival

of motor neuron; SOD1, superoxide dismutase 1.

Competing interests

JTD and MG are employees of iPierian, Inc. MAS declares that he has no

competing interests.

Authors’ contributions

The present review was written and edited by MAS, MG and JTD.

Acknowledgements

The authors would like to thank members of their laboratories, Berta

Strulovici, Irene Griswold-Prenner and Mike Shy for helpful discussions on this

manuscript. MAS is a fellow of the Inherited Neuropathies Consortium – Rare

Diseases Clinical Research Network – National Institutes of Health.

Author details1iPierian, Inc., 951 Gateway Blvd, South San Francisco, CA 94080, USA. 2Department of Neurology, Wayne State University School of Medicine, 421 E

Canfi eld, Room 3209, Detroit, MI 48201, USA.

Published: 21 September 2011

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doi:10.1186/scrt78Cite this article as: Saporta MA, et al.: Induced pluripotent stem cells in the study of neurological diseases. Stem Cell Research & Therapy 2011, 2:37.


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